There is quite a bit to learn about Lugers before you can make an educated judgement about what you're buying. The best initial investment is a lower priced pistol as Don mentioned, and investing in some books.
You can download, absolutely free, an extensive reference FAQ document from this site. You'll find the material a good basic introduction to Lugers, and find it useful when you start looking to buy one. It's not exhaustive - that is why they publish books - but it has quite a bit of accumulated Luger knowledge and lore...
http://forum.lugerforum.com/showthread.php?t=13121
Welcome to the forum! Marc
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.